An x-ray fluoroscopic apparatus and an x-ray image acquiring apparatus for measuring an x-ray image by using a two-dimensional x-ray detector are known techniques. There are a cone-beam CT for performing image acquisition while rotating an x-ray source and a two-dimensional x-ray detector around the subject, and a cone-beam CT for performing image acquisition while fixing an x-ray source and a two-dimensional x-ray detector and rotating the subject (Japanese Patent Application Laid-Open No. 10-192267: prior art 1).
In the cone-beam CT, by performing a correcting process on each of a plurality of images acquired by the rotation image acquisition, a set of projection data is obtained. By performing a three-dimensional reconstructing process by using a three-dimensional reconstruction algorithm on the set of projection data acquired, a three-dimensional image is obtained.
The three-dimensional reconstruction algorithm of CT is described in “Image Engineering” (by Shin Hasegawa, Corona Publishing Co., Ltd., pp. 195-199: prior art 2). As the three-dimensional reconstruction algorithm of cone-beam CT, a method such as Feldkamp method is used. Particularly, the Feldkamp method is described in “Practical cone beam algorithm” (L. A. Feldkamp, Journal of Optical Society of America, Vol. 1(6), pp. 612-619, 1984: prior art 3).
Two-dimensional x-ray detectors used for cone-beam CT include an XII-camera type x-ray detector obtained by combining an x-ray image intensifier (hereinbelow, abbreviated as XII) and a TV camera via an optical system (Japanese Patent Application Laid-Open No. 10-192267: prior art 1), and a flat-type x-ray detector.
The flat-type x-ray detector and the possibility of applying the detector to tomography is described in “Trend of Flat Panel Detector” (Kiyonari Inamura, Video Information, vol. 31(4), pp 125-130: prior art 4).
For example, the configuration of a plat-type x-ray detector in which sets each of an amorphous silicon diode (hereinbelow, abbreviated as a-SiPD) and a TFT are arranged in a square matrix and the sets and a fluorescent screen are directly combined is known. In a flat-panel type x-ray detector of this type, an x-ray is incident on the fluorescent screen, thereby generating fluorescence which enters the nearest a-SiPD and is converted into charges. The charges are accumulated until being read. In this example, the a-SiPD functions as a discrete sensing element.
An example of means for reducing scattering x-rays which are incident on the two-dimensional x-ray detector is a scattering x-ray shielding grid (hereinbelow, simply called anti-scatter grid). By disposing the anti-scatter grid on the front face of the two-dimensional x-ray detector, scattering rays can be reduced in x-ray fluoroscopy, x-ray image acquisition, and cone-beam CT image acquisition.
The anti-scatter grid has a stacked structure in which an x-ray transmitting member such as aluminum and an x-ray shielding material such as lead are alternately stacked. There is a cross anti-scatter grid in which two anti-scatter grids each having the stack structure are disposed so as to perpendicularly cross each other and integrated. It is known that an anti-scatter grid for blocking scattering x-rays for cone-beam CT image acquisition (Japanese Patent Application Laid-Open No. 9-149895: prior art 5).
A technique of binning which adds output signals of a plurality of sensing elements of a two-dimensional x-ray detector in a one-dimensional direction or two-dimensional direction is known (Japanese Patent Application Laid-Open No. 9-197051: prior art 6). It is known that, in an XII-camera type x-ray detector, the number of output pixels of a TV camera can be changed. The technique is known also in cone-beam CT image acquisition (Japanese Patent Application Laid-Open No. 11-226004: prior art 7).
The binning can realize an effect of decreasing the number of data of output signals and reducing time for outputting an image and an effect of shortening time required for data processing. After temporarily storing output signals of sensing elements into a memory, binning may be performed in a post process, thereby enabling time required for a computing process after the binning to be shortened.
Another known technique is that the pitch of x-ray shielding members of an anti-scatter grid projected on the surface of sensing elements of the flat-type x-ray detector with x-rays is set to a fraction of an integer of the arrangement pitch of sensing elements (Japanese Patent Application Laid-Open No. 9-75332: prior art 8).
There is also a known method of matching the position of an x-ray source and the position of an anti-scatter grid by using the shadow of an x-ray shielding member of an anti-scatter grid projected on the surface of sensing elements of a flat-type x-ray detector with x-rays (Japanese Patent Application Laid-Open No. 4-308809: prior art 9).
There is also a known method of aligning an x-ray source and a detector by using the shadow of an x-ray shielding member of an anti-scatter grid projected on the surface of sensing elements of a flat-type x-ray detector with x-rays (Japanese Patent Application Laid-Open No. 9-66054: prior art 10).
First, terms used for the following description will be explained.    (1) amorphous silicon photo diode: In the following description, it will be abbreviated as a-SiPD.    (2) scattering x-ray shielding grid: In the following description, it will be simply called an anti-scatter grid.    (3) flat-type x-ray detector: an x-ray detector in which sensing elements are arranged in a two-dimensional plane. In the following description, it will be simply called an x-ray detector.    (4) sensing element: In an example to be described below, a sensing element is constructed by a set of an a-SiPD and a TFT and a fluorescent layer. The sensing element includes a sensitive part in which an a-SiPD for changing fluorescence generated by the fluorescent layer with an x-ray into an electric signal is formed and a blind part which is the part other than the sensitive part.    (5) sensing surface: In an example to be described below, a sensing surface is a surface on which a plurality of a-SiPDs are formed in a flat-type x-ray detector.
In an x-ray measuring apparatus for diagnosis such as an x-ray fluoroscopic apparatus, an x-ray image acquiring apparatus, and a cone-beam CT apparatus for measuring an x-ray image by using an x-ray detector, improvement in a spatial resolution is required to improve diagnostic performance, so that the size of the sensing elements of the x-ray detector is being reduced. In the x-ray measuring apparatus for diagnosis, in addition to improvement in spatial resolution, reduction in an x-ray exposure amount is also required, so that higher sensitivity of x-ray detection is demanded.
In the case of using an x-ray detector, there is a problem such that deterioration in picture quality by scattering x-rays cannot be avoided. In the case of adjusting the pitch of x-ray shielding members of the anti-scatter grid to the pitch of sensing elements in order to reduce scattering x-rays, as the size of the sensing elements is reduced, the pitch of the x-ray shielding members becomes smaller. It causes deterioration in sensitivity by the anti-scatter grid and a problem arises such that exposure of x-rays has to be increased to obtain predetermined sensitivity. It can be considered that, when the size of the sensing element is reduced, the pitch of x-ray shielding members in the anti-scatter grid is set in correspondence with the pitch of a plurality of sensing element groups, thereby setting the pitch of the x-ray shielding members to be larger than that of sensing elements.
However, any of the above cases has a problem such that moiré occurs due to loss of the corresponding relation between the arrangement pitch of x-ray shielding members and the pitch in the sensing distribution of the x-ray detector. Particularly, since a sensing distribution characteristic is sharp in the x-ray detector, moiré tends to occur. In a cone-beam CT image, there is a problem such that degradation occurs in picture quality due to artifact caused by moiré and quantitativeness of a CT value deteriorates. For example, the inventor herein has found, as shown in FIG. 6, not only a ring artifact but also an artifact which has a plurality of curved parts derived from a ring and is generally very complicated. FIG. 6 shows an example of the artifact which appears in the center portion of a reconstructed image of a water phantom of a cylindrical shape having a large diameter.
In an x-ray measuring apparatus using an x-ray detector having small-sized sensing elements, it is desired to effectively reduce scattering x-rays and realize higher sensitivity and higher spatial resolution without deteriorating sensitivity.